In the W-CDMA system used as a communication system of a mobile communication terminal apparatus such as a mobile telephone today, all users use carriers having the same frequency.
However, when each mobile communication terminal apparatus transmits a signal to a base station at the same transmission level (transmission power) severally, a signal level from a mobile communication terminal apparatus nearer to the base station is larger than a signal level from a mobile communication terminal apparatus farther from the base station. Consequently, a problem of the difficulty of separating a signal transmitted from the mobile communication terminal apparatus farther from the base station by the base station occurs (distance problem).
Accordingly, a base station supporting the W-CDMA system performs communication by controlling the transmission level of each mobile communication terminal apparatus severally to the minimum necessary in accordance with the adaptive transmission power control based on measurement values of SIR (signal power to interference power ratios).
To put it concretely, the base station performs the despread processing of a signal received from each mobile communication terminal apparatus, and performs the RAKE synthesis of the signal. Then, the base station measures the above-described SIR measurement value. When the measured SIR measurement value is larger than a predetermined value (target value), the base station transmits a control command for lowering the transmission level to the mobile communication terminal apparatus. Moreover, in a case where the measured SIR measurement value is smaller than the above-described target value, the base station transmits a control command for raising the transmission level to the mobile communication terminal apparatus.
The mobile communication terminal apparatus adjusts its transmission level in accordance with the control command, and performs the transmission of a signal to the base station at the adjusted transmission level.
FIG. 7 shows a circuit diagram of a power-variable amplifying unit provided in a mobile communication terminal apparatus for performing such adaptive transmission power control.
As it is apparent from FIG. 7, the power-variable amplifying unit is an amplifying unit in three-stage configuration composed of a variable gain amplifier 101 capable of changing its gain, a fixed gain driver amplifier 102, and a fixed gain power amplifier 103, all connected in cascade.
A bias voltage (power supply voltage) is supplied to each of the amplifiers 101-103 from a power supply control unit, and each of the amplifiers 101-103 is in its driven state during a high level period of a signal for changing the transmission system to be in its driven state (active state) from the control unit (CPU) of the mobile communication terminal apparatus.
Moreover, the variable gain amplifier 101 is configured in order that the gain thereof may be variably controlled by a control voltage supplied from the CPU in accordance with the control command from the above-described base station.
When a transmit signal processed by spread modulation processing is supplied to the power-variable amplifying unit configured as above from a spread modulation processing unit through an input terminal 100, first the variable gain amplifier 101 amplifies the transmit signal by the gain in accordance with the control command from the above-described base station, and successively the driver amplifier 102 and the power amplifier 103, each having a fixed gain, severally amplify the transmit signal from the variable gain amplifier 101. Then, the transmit signal is transmitted to the above-described base station from an output terminal 104 through an antenna. Thereby, the above-described distance problem can be solved.
Incidentally, the following Japanese Patent Application Publication (KOKAI) No. Hei 9-116357 discloses a prior art document of such a gain amplifying circuit.
Patent Document: Japanese Patent Application Publication (KOKAI) No. Hei 9-116357 (see pp. 3-4, FIG. 1)
Here, FIG. 8A shows a characteristic diagram showing a relation between the control voltage and the gain of the above-described variable gain amplifier 101, and FIG. 8B shows a characteristic diagram showing a relation between the current value supplied to the above-described driver amplifier 102 and the control voltage of the above-described variable gain amplifier 101.
In FIG. 8A, the abscissa axis indicates the control voltage (V) supplied from the CPU to the variable gain amplifier 101, and the ordinate axis indicates the gain (db) of the variable gain amplifier 101 changed by the control voltage. As shown in FIG. 8A, the gain of the variable gain amplifier 101 gradually increases as the control voltage to be applied to the variable gain amplifier 101 is made to be larger.
On the other hand, in FIG. 8B, the abscissa axis indicates the above-described control voltage (V) to be supplied from the CPU to the variable-gain amplifier 101, and the ordinate axis indicates the current value (I) to be supplied to the driver amplifier 102.
The driver amplifier 102 is designed in order that the characteristics of the power-variable amplifying unit at the time of the maximum output of the aforementioned power-variable amplifying unit may satisfy an adjacent channel power leakage ratio as well as the above-described SIR (signal power to interference power ratio), and the driver amplifier 102 is configured to consume the same current quantity as that at the time of the maximum output even at the time of a low output, as shown in FIG. 8B. This means that the driver amplifier 102 has an excessive linear characteristic at the time of the low output.
Because the mobile communication terminal apparatus is used in a carried state, it is important to elongate the usable time (including a continuous talking time and a continuous waiting time).
However, as described above, because the driver amplifier 102 is configured to consume the same current quantity as that at the time of the maximum output despite at the time of a low output, the driver amplifier 102 consumes much waste power, and it is a primary factor of checking the elongation of the usable time of the mobile communication terminal apparatus.
The present invention was made in view of the above-mentioned problems, and aims to provide a communication terminal apparatus and an amplifying circuit, both capable of achieving the reduction of power consumption, and of elongating a usable time of the mobile communication terminal apparatus to which the present invention is applied.